This invention relates to fluid spraying and more particularly to an improved electrostatic spray painting system capable of automatic painting work pieces with different colored electrically conductive paints.
In production lines and similar applications, it is desirable to have a paint system capable of painting successive work pieces such as automotive bodies different colors as they are conveyed past a spray station. As a consequence, both manual and automatic systems have been developed for changing color as successive work pieces are painted. In many applications, it is also desirable to use an electrostatic coating system. Electrostatic spray painting has many advantages including producing a more uniform coating on irregular surfaces and reducing the amount of paint needed to coat a work piece. Many problems have occurred in attempting to combine a color change system with an electrostatic system. Many of the problems are aggravated when an electrically conductive paint is to be sprayed.
When an electrostatic spray system is operated with an electrically conductive paint, it is necessary to electrically isolate from ground the entire column of paint from the spray gun to its supply tank or source. In a color change system, it is further necessary to isolate from ground each of the individual colored paints. Where the system permits all of the paint to be charged from the spray gun back to their source, it is not possible to perform maintainance work on any portion of the system while the spray gun is in operation. For example, while the system is painting work pieces with red paint, it is not possible to fill a different color tank, such as the green paint tank, with additional paint. Problems also occur from electrical capacitance of a color change system. Where tanks of different colored paints, paint hoses and color selection valves are added to a system, the electrical capacitance is greatly increased. This higher electrical load is often sufficient to prevent the high voltage power supply from maintaining a desired potential at the spray gun. Furthermore, the higher capacitance will store more electrical energy and, therefore, present a greater hazard to workmen in the vicinity of the spray system.
U.S. Pat. No. 4,085,892 discloses an electrostatic spray system capable of handling a purality of different colored coating materials which are electrically conductive. This system operates with two sub-systems, one of which includes a source of each of the different colored paints maintained at ground potential and the other of which maintains tanks of the paints which are isolated from ground. The isolated paints are all connected through paint hoses and manifolding to the spray gun and are all charged to a high voltage. Isolation is maintained between the two systems by pumping relatively small masses of paint or bursts of paints from the ground system across a relatively large air space into the electrically charged tanks. However, this system has a large electrical capacitance and, therefore, presents a high electrical load on the high voltage power supply. Also, maintainance personnel cannot work on the ungrounded tanks for any of the colors when the system is spraying a different color since all tanks are charged during spraying.